the way wrist moves around the contact point, it's most natural to hit flattish shot than loopier shot even in 1hbh as in 2hbh. don't try too hard to achieve that loopiness and topspin. instead try to raise the contact point and drive through more with correct footwork for consistency. recognize each time your footwork is incorrect and remind yourself until correct footwork becomes habit.

My one hander was always flattish and deep until I realised to loosen my grip, change it to more eastern and stay relaxed. For me, this was the major key. There's lots of other nibble things, like footwork, stepping into the shot, timing but you will figure those by simply hitting a lot of balls.

2) It's all in the shoulders. Once you start the forward motion of the swing, lock your torso in place and don't let the shoulders open up until after contact. I can't stress this enough. Once you've hit the ball, you can release. But for consistency with your contact, hold your shoulders steady.
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One of the issues I had in the past was my upper body as I used to lean back during the forward swing. As it happens, it was probably how I got my left hand involved in the swing (I am right handed). I never really got the Federer type left arm extension and I tend to lean back to compensate -- bad habit, I know. I begun to correct this issue only more recently, but then I switched to a two handed backhand and never really got to work on it as much as it would be required.

As you say here, once you start moving too much things around, you're spraying balls all over the place. I'll bother working on this, thanks.

For other pointers, I would like to know what people think of how Federer really exaggerates certain things, such as the left arm extension. Many pros seem to be a LOT less "shy" about doing these sorts of details than amateurs, especially for one handed backhand players.